Additive manufacturing techniques and processes generally involve the buildup of one or more materials to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), additive manufacturing encompasses various manufacturing and prototyping techniques known under a variety of names, including freeform fabrication, 3D printing, rapid prototyping/tooling, etc. Additive manufacturing techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer-aided design (CAD) model.
One such process, commonly referred to as Fused Deposition Modeling (FDM), comprises a process of melting a very thin layer of a flowable material (e.g., a thermoplastic material), and applying this material in layers to produce a final part. This is commonly accomplished by passing a continuous thin filament of thermoplastic material through a heated nozzle, which melts the thermoplastic material and applies it to the structure being printed. The heated material then is applied to the existing structure in thin layers, melting and fusing with the existing material to produce a solid finished product.
A common method of additive manufacturing, or 3D printing, generally includes forming and extruding a bead of flowable material (e.g., molten thermoplastic), applying the bead of material in a strata of layers to form a facsimile of an article, and machining such facsimile to produce an end product. Such a process is generally achieved by means of an extruder mounted on a computer numeric controlled (CNC) machine with controlled motion along at least the X, Y, and Z-axes. In some cases, the flowable material, such as, e.g., molten thermoplastic material, may be infused with a reinforcing material (e.g., strands of fiber) to enhance the material's strength.
The flowable material, while generally hot and pliable, may be deposited upon a substrate (e.g., a mold), pressed down or otherwise flattened to some extent, and leveled to a consistent thickness, preferably by means of a tangentially compensated roller mechanism. The flattening process may aid in fusing a new layer of the flowable material to the previously deposited layer of the flowable material. In some instances, an oscillating plate may be used to flatten the bead of flowable material to a desired thickness, thus effecting fusion to the previously deposited layer of flowable material. The deposition process may be repeated so that each successive layer of flowable material is deposited upon an existing layer to build up and manufacture a desired component structure. When executed properly, the new layer of flowable material may be deposited at a temperature sufficient enough to allow a new layer of such material to melt and fuse with a previously deposited layer, thus producing a solid part.
The process of 3D-printing a part, which utilizes a large print bead to achieve an accurate final size and shape, requires a two-step process. This two-step process, commonly referred to as near-net-shape, begins by printing a part to a size slightly larger than needed, then machining, milling or routing the part to the final size and shape. The additional time required to trim the part to final size is more than compensated for by the much faster printing process.
In the practice of the aforementioned process, a major deficiency has been noted. In creating parts with successive layers, the layers must be applied in uniform, smooth beads with no trapped air between layers. In applying a successive layer of material upon an existing layer, the existing layer must be leveled smoothly in order to effectively bond with the successive layer. The successive layer of material has to be leveled smoothly and trapped air must be pressed out between the layers. The layers must be of uniform width, height, and shape, in order to produce consistent parts. Also, the flattening device must be able to navigate corners without gouging in, or dragging the flowable material. Smooth layers allow for better bonding between layers, resulting in better strength characteristics in the finished part. Uniform layers allow for consistent bonding of layers, plus less machining time in order to get a smooth part. Air in or between layers can cause voids in the part when machined, which can weaken the bond, and render the part unusable.
In past attempts to address the aforementioned concerns, a number of different methods have been attempted. One such attempted method involves the use of an oscillating plate for tamping the bead to achieve both leveling and bonding. Such a device, however, does not create a smooth bead of uniform width, and therefore requires extra machining, among other problems. While the use of a roller is the preferred method for achieving a smooth and well-bonded strata of layers, attempts by the prior art to employ such a method have resulted in unsatisfactory results.
Another method that has been employed is the use of a grooved roller. A grooved roller, however, does not create a smooth bead, nor does it remove the trapped air between the layers. Attempts to employ a smooth, straight roller have been met with some success; however this method has likewise given rise to unsatisfactory results. The desired compression roller must be somewhat wider than the final compressed surface of the deposited layer that it is flattening. This is due to a number of factors intrinsic to the process, including coverage requirements when negotiating curves and corners in the deposition process.
Another inherent characteristic of the additive manufacturing process is the slight decompression of the deposited layer, which occurs immediately after the compression roller passes over the bead of molten material. Such action results in the surface of the final flattened layer rising up, and remaining slightly higher than the bottom of the compression roller. When an applicator head rotates to execute a change in tool-path direction, the edges, as well as the outer regions of the roller tend to engage the surface of the final flattened layer, since it is slightly higher than the bottom of the roller, resulting in the roller gouging and dragging the deposited material during the transition. A similar problem occurs when depositing a layer adjacent to a previously deposited layer. In such a case, the existing layer is again, slightly higher than the bottom of the roller, the result of which is the same type of problem encountered during directional transition. It is therefore desirable to provide a compression roller of a design that will eliminate, or greatly mitigate the negative aspects of a typical, straight cylindrical roller.